Total War: Warhammer 2
I've personally not had this problem. But the way I've had this option consistently work for ms is to, while in mid-battle or in deployment phase, I enable the option and click on all the units I want to not take casualties, one by one. You can leave the option on for the duration of the battle with no side effects.
I've only ever lost 3-8 units, and that was just because I may have missed clicking on a unit.
But the unlimited mana option has always been a bit fiddly in my experience as far as I know. I rarely use it as I personally find I don't need it with a fully maxed out lord or wizard. So it's not a concern for me personally.